Personal computer users, businesses, and public offices are faced with a deluge of data in the form of digital information. The question of how to preserve this data for the next decade, and for the ages, has yet to be answered. The most common data backup includes storing the data on a writable compact disk CD, CD-R (compact disk, write once read-only memory), CD-ROM (compact disk, read-only-memory), or a CD-RW, short for CD-ReWritable disk. A CD-ROM is an adaptation of the CD and is designed to store data in the form of text and graphics, as well as sound. A CD-RW is a type of CD disk that enables a user to write onto the disk multiple times. A CD-R comprises an organic layer sandwiched between a transparent base and a reflective layer. When heated by a focused laser, the dye layer melts and forms a series of pits, which are readable by a laser beam as 0's and 1's.
The technology behind a CD-RW is known as optical phase-change, an optical storage technology in which data is written with a laser that changes dots on the disk between amorphous and crystalline states. Phase change is a type of CD recording technology that enables the disks to be written, erased, and rewritten through the use of a layer of a special material for the recording layer—the phase change layer—that can be changed repeatedly from an amorphous (formless) to a crystalline state. The crystalline areas allow the metalized layer to reflect the laser beam better, while the non-crystalline portion absorbs the laser beam, so the beam is not reflected. An optical head reads data by detecting the difference in reflected light from amorphous and crystalline dots.
During writing, a focused laser beam selectively heats areas of the phase-change material above the melting temperature, so all the atoms in this area can rapidly rearrange. The recording phase-change layer is sandwiched between dielectric layers that draw excess heat from the phase-change layer during the writing process. Then, if cooled sufficiently quickly, the random state is “frozen-in,” and the so-called amorphous state is obtained. The amorphous version of the material has different reflection properties where the laser dot was written, resulting in a recognizable CD surface. Writing takes place in a single pass of the focused laser beam, which is referred to as “direct overwriting,” and can be repeated several thousand times per disk. Once the data has been burned, the amorphous areas reflect less light, enabling a “Read Power” laser beam to detect the difference between the lands and the pits on the disk. The recorded tracks on a CD-RW disk are read in the same way as regular CD tracks. That is, by detecting transitions between low and high reflectance, and measuring the length of the periods between the transitions. The only difference is that the reflectance is lower than for regular CDs.
A digital versatile disk (DVD) provides an optical disk technology that allows for much greater storage as compared with CDs. With reference to FIGS. 1 and 2, a DVD's sevenfold increase in-data capacity over the CD has been largely achieved by tightening the tolerances throughout the predecessor CD system. The tracks on the DVD are placed closer together, thereby allowing more tracks per disk than found on CDs. As shown in FIG. 2, the DVD track pitch 4 is reduced to 0.74 microns, less than half of CD's 1.6 micron track pitch 2, as shown in FIG. 1. The pits 6, in which the data is stored, are also a lot smaller, allowing more pits per track. The minimum pit length 10 of a single layer DVD is 0.4 microns, as compared to 0.83 microns pit length 8 for a CD. With the number of pits having a direct bearing on capacity levels, the DVD's reduced track pitch and pit size alone give DVD ROM disks four times the storage capacity of CDs. The packing of as many pits as possible onto a disk is, however, the simple part. The real technological breakthrough of the DVD was with its laser. Smaller pits mean that the laser has to produce a smaller spot, and the DVD achieves this by reducing the laser's wavelength from the 780 nanometers infrared light of a standard CD, to 635 nm or 650 nm red light.
The first-generation CD players used a 780 nm AlGaAs laser diode developed in the early 1980s. With this technology, a CD-ROM stored about 650 Mbytes of information. The shortest wavelength commercially-viable device that was made in this system was about 750 nm. Further shortening of the wavelength called for a different material, and in the late 1980s red-emitting laser diodes were developed in the AlGalnP system, grown lattice-matched on a GaAs substrate. This material has provided the laser for new DVDs, which store about 4.7 Gbytes of information. Different materials are used to make a laser emit blue light, e.g., at wavelengths in the range of 430 nm to 480 nm. One technique reported has been laser action at 77K from a GaN-based device by researchers at Nichia Chemical Industries in Japan. Nichia announced pulsed room temperature operation at the end of 1995, and continuous operation in early 1997. By August 1997 the room temperature operating life had reached 300 hours. Based on accelerated life-testing at elevated temperatures, Nichia reported in 1999 a room temperature operating life of about 10000 hours at room temperature. A wide variety of solid state laser diodes are now available for use in CD-ROM or CD-ROM like technology.
While current optical disk technologies such as DVD, DVD±R, DVD±RW, and DVD-RAM use a red laser to read and write data, a new format uses a blue-violet laser, sometimes referred to as Blu-ray. The benefit of using a blue-violet laser (405 nm) is that it has a shorter wavelength than a red laser (650 nm), which makes it possible to focus the laser spot with even greater precision. This allows data to be packed more tightly and stored in less space, so it is possible to fit more data on the disk even though it is the same size as a CD or DVD. This together with the change of numerical aperture to 0.85 is what enables Blu-ray Disks to hold 25 GB. Blu-ray technology should become available in the 2005 to 2006 time frame. Some new techniques proposed for archival storage have included “a polymer/semiconductor write-once read-many-times memory” and some “novel concepts for mass storage of archival data” using energetic beams of heavy ions to produce radiation damage in thin layers of insulators.
Current CD-ROM memories based on changes in organic dyes or phase changes in layers may degrade over time and become unreadable. Although at normal temperature and humidity the life span of CD could be in excess of 100 years, the life span of data on a CD recorded with a CD burner could be as little as five years if it is exposed to extremes in humidity or temperature. And, if an unprotected CD is scratched it can become unusable. What is needed is a data storage medium that can provide greater long-term stability for the stored data.